The term "foaming agent" or "blowing agent" is used to describe any substance which alone or in combination with other substances is capable of producing a cellular structure in a plastic or rubber mass. Thus, the term includes gases which expand when pressure is released, soluble solids that leave pores when leached out, liquids which develop cells when they change to gases, and chemical agents that decompose or react under the influence of heat to form a gas.
Known liquid foaming agents include certain aliphatic and halogenated hydrocarbons, low boiling alcohol, ethers, ketones, and aromatic hydrocarbons. Chemical foaming agents range from simple salts such as ammonium or sodium bicarbonate to complex nitrogen releasing agents, of which azobisformamide is an important example.
Foaming agents, also known as chemical blowing agents (CBA's) can be utilized in all conventional plastics processes, such as extrusion, calendering, injection molding, coating, expansion casting, and rotational molding.
Recognized advantages of foamed plastics include reduction in density, savings in material costs, improved electrical and thermal insulative properties, increased strength to weight ratio, and the elimination of shrinkage, warpage, and sink marks in injection molded parts. Foamed plastic products include such diverse items as vinyl flooring, insulated food containers, structural foam furniture, business machine housings, simulated leather, and foamed core pipe.
Foaming agents are generally classified as physical or chemical. Chemical foaming agents (generally solids) undergo a chemical transformation when producing gas, while physical foaming agents undergo a generally reversible physical change of state, e.g., vaporization.
The two major categories of physical foaming agents include both liquids and gases. The gas most often is compressed nitrogen. In injection molding processes which utilize physical foaming agents, the gas is injected under high pressure directly into the polymer during plastication, and the mixed polymer and gas are metered into the mold. When the pressure is relieved, the gas becomes less soluble in the polymer and expands, forming the cellular structure. Nucleating agents, in the form of finely divided powders and chemical foaming agents, sometimes are used with the gas to yield a finer cell structure.
The preference for nitrogen is due to the fact that nitrogen is inert, nonflammable, leaves no residue in the polymer, and is not temperature restrictive. However, the use of processes involving nitrogen is limited in the industry due to the requirement of licensed specialized equipment. Moreover, the process tends to produce foams with poorer performance and appearance than those produced by the use of exothermic chemical blowing compounds.
Liquid physical foaming agents include volatile liquids which produce gas through vaporization. Common liquid physical foaming agents generally include shortchain aliphatic hydrocarbons (C.sub.5 to C.sub.7) and their chlorinated and fluorinated analogs. Liquid physical foaming agents may be used over a wide temperature range in low pressure and atmospheric processes, and are widely used to produce low density thermoplastics, such as foamed polystyrene, and thermoset polymers, such as polyesters, epoxy, and polyurethane foam systems.
Chemical foaming agents commonly referred to as blowing agents are generally solids that liberate gas(es) by means of a chemical reaction or decomposition when heated. They are necessarily selected for specific applications or processes based on their decomposition temperatures. In this regard, it is important to match the decomposition temperature with the processing temperature of the polymer to be foamed. If the polymer processes at temperatures below that of the chemical foaming agent, little or no foaming will occur. If the process temperature is significantly above the foaming agent's decomposition temperature, poor (overblown, ruptured) cell structure and surface skin quality will likely result.
Chemical foaming or blowing agents may be either inorganic or organic. The most common inorganic foaming agent is sodium bicarbonate. Sodium bicarbonate is inexpensive, nonflammable and begins to decompose at a low temperature; however, it is used only to a very limited extent in thermoplastics. Differential thermal analysis has shown that sodium bicarbonate decomposes over a broad temperature range and this range is endothermic, contributes to an open cell structure in the finished product, and the released gas (carbon dioxide) diffuses through the polymer at a much greater rate than nitrogen gas.
Presently used chemical foaming or blowing agents are mostly mixtures of sodium bicarbonate and sodium hydrogen citrate. The citrate is incorporated together with the sodium bicarbonate in order to facilitate a complete acid assisted decomposition reaction to produce carbon dioxide gas. The mixture is also available in a variety of polymer concentrates at various loadings. The mixture is also available as a hydrophobized acid and carbonate which is a free flowing non-dusting powder.
The major advantages associated with utilizing endothermic foaming or blowing agents over their exothermic counterparts include short degassing cycles, small cells, smooth surfaces, weight reductions, reduced cycle times, foamed products which have promptly paintable surfaces, the foaming process is odorless, and the components of the foaming agents are generally regarded as environmentally safe.
The major disadvantage of existing acid/carbonate systems involves the formation of corrosion on the process equipment. This corrosion is attributed to the action of the citric acid and sodium bicarbonate on the lesser grades of steel used in some equipment. Another disadvantage associated with existing acid/carbonate blowing agents is premature reaction with water or moisture of the blowing agents when they are associated with polymeric reaction mixtures. This premature reaction when occurring prior to a foaming process detrimentally effects the final products.
Organic foaming or blowing agents can be utilized in most polymer applications and processes. These compounds release gas (usually nitrogen and/or ammonia) over a narrow temperature range. The rate of gas evolution for a given chemical foaming or blowing agent is determined by a temperature and time relationship. Applications for chemical foaming agents are generally divided into three areas: low, medium and high temperature processing polymers. There are numerous organic foaming agents available that decompose at various temperatures.
Foamed products are produced by a number of molding processes including injection molding, blow molding, rotational molding, and the like. In these molding processes weld lines (also known as knit or flow lines) are produced whenever two melt fronts collide, i.e., whenever two streams of molten plastic meet and cool in a mold. These resulting weld lines form areas which have different mechanical properties thereby creating structural problems in the foam product industry.
It has been discovered that fillers and/or pigments, especially platelet and/or fillers, e.g., mica and talc, create very weak weld lines in either injection or blow molding processes. N. Burditt et al, "The Knit-line Strength of Mica-filled Polypropylene", Plastics Compounding, March/April 1985, pp. 62-66, discusses knit-line problems involving two surface treated phlogopite micas. Knit-line strength problems were found to be due to a combination of flake orientation and delamination.
M. Christie, "Toughening Weld Lines of Micareinforced PP Parts", Plastics Engineering, July 1986, pp. 41-43 and G. Brewer, "A Technique for Strengthening Weldlines in Thermoplastic Parts", ANTEC, pp. 252-254 (1987) both discuss proposed methods for strengthening weld lines in molded foam products. These proposed methods include the use of coupling agents, reducing flake size and thickness of fillers, annealing welded parts, mechanical milling of weld lines and solvent treatments to relieve mixing orientation.
It is necessary to include fillers and/or pigments such as mica and talc in foamed products in order to enhance physical properties of foamed products Therefore, a great need exists in the foamed product industry for a procedure or process by which foamed products containing pigments and/or fillers having strong weld lines can be produced.
The present invention provides such a process by which foamed products containing pigments and/or fillers can be molded with strong weld lines.
The present invention further provides for endothermic foaming or blowing agents which are highly stable and which do not prematurely react with water or moisture.